The technical field generally relates to diagnosing a NOx sensor, and more particularly but not exclusively relates to detecting a response difference between two NOx sensors on each side of a catalyst having ammonia storage capacity. Modern internal combustion engines often utilize aftertreatment systems to achieve emissions regulatory targets. One aftertreatment system is a NOx reduction device, including a catalyst for a selective catalytic reduction (SCR) system. It is helpful for controls, and in some cases dictated by regulation, that a failed or off-nominal NOx sensor be detected, allowing the control scheme to use an alternate NOx determination and/or to set a failure indicator. NOx sensors that are commercially viable for field use with an internal combustion engine have an interference with ammonia (NH3), erroneously detecting a significant percentage (80% or higher) of NH3 as NOx. Further, reactions within the NOx reduction device cause a difference to occur from the NOx in to the NOx out that is not attributable to the sensors. Therefore, merely comparing sensor signals during engine operations will not generally allow a determination of a failed or off-nominal sensor. Therefore, further technological developments are desirable in this area.